ABSTRACT This work looks at the antibacterial efficacy of Gracilaria vermiculophylla and Ulva flexuosa aqueous extracts when added to the feed of Penaeus vannamei in an experimental infection with Vibrio parahaemolyticus (M0904AHPND+strain). Results show that the minimum inhibitory concentration (MIC) for both extracts was 50 mg mL-1, with inhibition zones of 18.00 ± 0.60 mm for G. vermiculophylla and 14.00 ± 0.29 mm for U. flexuosa. G. vermiculophylla gave a higher total content of phenolic compounds (10.58 ± 2.31 mg GAE g-1) (gallic acid equivalent) and lower flavonoid content (10.32 ± 0.73 mg QE g-1) in comparison to U. flexuosa, while using ABTS (2,2-diphenyl-1-picrylhydrazyl) and ferric reducing antioxidant power (FRAP) to measure antioxidant capacity showed that G. vermiculophylla gave a better result than U. flexuosa. The challenge with V. parahaemolyticus resulted in 67% survival for organisms fed G. vermiculophylla and 60% for those fed U. flexuosa, resulting in twice the amount of survival as opposed to 30% in the positive control at 24 h post-infection. Histopathological alterations in the hepatopancreas with hemocytic infiltration within the intertubular connective tissue were observed. Also, tubules with severe cell detachment and tubular atrophy were detected in the positive control organisms, and organisms treated with macroalgae only had vermiform structures in the tubular lumen, cell detachment, and infiltration hemolymph in intertubular connective tissue. According to the analysis of the studied variables, it can be concluded that the aqueous extracts of these macroalgae are a promising alternative for V. parahaemolyticus (M0904AHPND+strain) control in shrimp culture.